As an acrylonitrile-based precursor fiber for a carbon fiber, for the purpose of obtaining a carbon fiber having high strength and high elasticity modulus, predominantly a so-called small tow of 3,000 to 20,000 filaments which tow scarcely suffers filament breaking and fluff generation and is excellent in quality has hitherto been produced; a carbon fiber produced from such a precursor fiber has been used in various fields such as the aerospace field and the sport field.
A precursor fiber for producing a carbon fiber is subjected to a flame retarding treatment in which, in advance of carbonization, the precursor fiber is heated in an oxidative atmosphere set at 200 to 350° C. The flame retarding treatment generates a reaction heat, and hence heat tends to be stored inside the fiber tow. When excessive heat storage occurs inside the fiber tow, yarn breaking and fusion bonding between fibers tend to be generated. Accordingly, it is necessary to suppress the heat storage due to the reaction heat as much as possible. For the purpose of suppressing the heat storage, the thickness of the fiber tow fed to a flame retarding oven is inevitably made to be equal to or less than a certain thickness; thus, a constraint is imposed on the thickness of the fiber tow, and accordingly constitutes factors that degrade the productivity and simultaneously raises the production cost.
For the purpose of solving the above described problems, for example, Patent Document 1 (Japanese Patent Laid-Open No. 10-121325) discloses a precursor fiber tow for a carbon fiber which tow maintains a form of a tow when housed in a container, but has a widthwise dividing capability to divide the tow into a plurality of small tows when the tow is taken out, from the container, to be used. For the purpose of producing the fiber tow having this dividing capability, a plurality of spun yarns (fibers) are divided into a plurality of groups each having a predetermined number of yarns, and the plurality of groups are made to travel parallel in this divided condition, made to pass through a fiber-making step and a finishing oiling agent imparting step, and transferred to a crimp imparting step involving a crimper. The crimp imparting carries out bundling of a predetermined number of the plurality of groups into a form of a tow. When the crimp imparting step is not applied, individual small tows each are made to contain water in a content of 10% or more and 50% or less.
In the bundled form, the yarns at the selvage of each of the yarn groups, each having a small tow form, are made to obliquely cross each other over approximately 1 mm to be mutually weakly intermingled, and thus, a single tow form made of a plurality of yarn groups is maintained. The intermingle based on the oblique crossing of the yarns at the selvage of each of the yarn groups is weak, and hence, when the bundled form is transferred to and used in a carbon fiber producing step after having been maintained in a single tow form, easy division into individual yarn groups from the selvage is made possible, and the bundled fiber bundle is housed in a container as it is in a condition capable of being divided into small tows.
The precursor fiber bundle, having dividing capability, for a carbon fiber housed in a container are divided into the above described individual small tows in a dividing step before being guided into the flame retarding oven. This division is to be carried out, for example, by using a grooved roll or a dividing guide bar. The small tows are mutually bundled by being weakly intermingled at the selvages thereof, and hence this division can be extremely easily carried out in such a way that breakage or fuzz generation scarcely occurs. The individual tows divided into small tow forms each having a predetermined size or less are guided into the flame retarding step to be subjected to the flame retarding treatment. In this treatment, it is stated that the small tows are subjected to the flame retarding treatment as they are in the divided condition, so that the excess heat storage is not generated, and accordingly the breakage or the fusion bonding between filaments is prevented.
The mechanism for imparting the bundled fiber bundle the dividing capability into small tows, according to above described Patent Document 1, is stated to be based on the intermingling due to oblique crossing of each fiber located at the selvages of the small tows; however, with a degree of intermingle of 1 to 10 m−1 at the dividing portions in the small tows, when division into small tows is carried out by a dividing means in advance of being guided into the flame retarding step, single yarn breaking is possibly caused and the quality of the carbon fiber is possibly affected thereby. Furthermore, in Patent Document 1, as means for intermingling small tows with each other, there is disclosed only a method that is based on the imparting of the crimp where a form of a tow is maintained by crossing the yarns at the selvages of the individual small tows obliquely with each other to weakly intermingle the yarns. In the case of such a crimped tow, when the crimped tow is guided as it is into the flame retarding step involved in a production process of a carbon fiber, it is difficult to uniformly straighten the crimps over the whole range of the tow to provide a predetermined elongation. Consequently, there may occur unevenness in the basis weight (weight per unit length) and the fineness of the obtained carbon fiber, and the quality of the obtained carbon fiber is possibly thereby affected. Under these circumstances, a crimp removing means is required in advance of the flame retarding step, which increases the equipment space, impedes work saving and significantly affects the productivity.
On the other hand, in aforementioned Patent Document 1, for the case of a form of a straight tow without imparted crimp, it is only described that the moisture percentage thereof is from 10 to 50%. In other words, there is described only a mechanism that the surface tension due to the moisture bundles the small tows to maintain a form of a single tow. With such moisture percentage, the surface tension due to the water within the tow serves to maintain the bent shape formed when folded and housed in a container, and consequently when fed to the production step of the carbon fiber, the tow is fed as it still has the bent shape and the oblique disposition of the filaments within the tow caused by the bent shape, the quality of the obtained carbon fiber is impaired, or sometimes the bent shape turns into twisted shape, and there is a fear that excessive heat storage is caused in such twisted portion in the flame retarding step.
Furthermore, irrespective as to whether or not a bundled fiber bundle is made to pass through a crimper, the bundled fiber bundle is required to be divided into small tows each having a predetermined thickness before the bundled fiber bundle is guided into a firing step after the bundle is taken out from a container; thus, a dividing device is required to be installed purposely, which increases the equipment space, impedes work saving and significantly affects the productivity.
Application of carbon fiber is being expanding over common industrial fields including automobiles, civil engineering, construction and energy. Accordingly, there are strong demands for supply of large tow carbon fiber high in strength, high in elastic modulus, high in grade and high in quality as well as large tow carbon fiber lower in price and excellent in productivity. For example, Patent Documents 2 and 3 disclose production methods of a large tow carbon fiber and a carbon fiber precursor fiber bundle; however, the carbon fiber disclosed in either of these Documents does not attain strength to a sufficient extent, and as affairs now stand, does not reach the strand strength and the elastic modulus comparable to those of a conventional small tow having the number of filaments of 12,000 or less.    Patent Document 1: Japanese Patent Laid-Open No. 10-121325    Patent Document 2: Japanese Patent Laid-Open No. 11-189913    Patent Document 3: Japanese Patent Laid-Open No. 2001-181925